Currently, searches for target-specific therapeutic and prophylactic compounds that have the ability to enhance or inhibit angiogenesis activity, enhance or inhibit cell death activity, and/or exhibit low toxicity comprise three major focuses of drug discovery and development. Angiogenesis plays an important role not only in the further development of the embryonic vasculature, but also in many post-natal processes, such as wound healing and tissue and organ regeneration. Angiogenesis has also been identified as a critical process for solid tumor growth. Furthermore, uncontrolled blood cell proliferation and excessive angiogenesis have been shown to constitute significant pathogenic components in numerous diseases, including rheumatoid arthritis, atherosclerosis, diabetes mellitus, retinopathies, psoriasis, and retrolental fibroplasia.
Methods of screening agents for an ability to inhibit or enhance angiogenesis activity would be useful in identifying those agents that would be effective in therapeutic or prophylactic treatment of a variety of diseases involving angiogenesis processes. For example, angiogenesis inhibition is a powerful potential approach for ameliorating cancer (Folkman, New Eng. J. Med. 333:1757–1763 (1995); Kerbel, Nature 390:355 (1997)) and for reversing blood vessel loss associated with tissue ischemia, such as diabetic retinopathy (Bonn, Lancet 348:604 (1996); Breier et al., Haemist. 78(1):678–683 (1997). It appears that anti-angiogenic therapies do not induce acquired drug resistance (Boehm et al., Nature 390:404–407 (1997))—a major problem with current cancer therapies. However, few therapeutic candidate molecules exist. It would therefore be desirable to provide methods of identifying compounds that inhibit angiogenesis and have therapeutic activities against diseases that would benefit from angiogenesis inhibition, such as cancer and diabetes. Similarly, methods of screening for compounds that enhance angiogenesis by stimulating blood vessel formation would be advantageous for use in minimally invasive approaches for improving circulatory function in various diseases, such as coronary artery disease, congestive heart failure, peripheral arterial disease, and peripheral venous disease. Unfortunately, many current assays for angiogenesis do not permit in vivo assessment of compounds or their side effects in whole animal models, or in multiple tissues or organs of animal models simultaneously and over time. In addition, many current assays for angiogenesis activity are not suitable for rapid, automated, or extensive compound screening, particularly screening of compound libraries containing numerous compounds, due to their complexity.
The search for compounds that can regulate promote or inhibit cell death has also been of vital interest. Necrosis and apoptosis are two known types of cell death. Necrosis involves the pathologic death of living tissue in a subject due to non-physiological injury to cells of the tissue. Apoptosis, which involves programmed cell death, is a physiological process that ensures that an equilibrium is maintained between cell proliferation and cell differentiation in most self-renewing tissues of multicellular organisms. Regulation of cell death activity (in particular, apoptosis) constitutes an important mechanism in therapeutic and prophylactic approaches to many diseases, including, e.g., cancer and organ transplantation. Existing models for assessing apoptosis activity include the nematode worm, C. elegans. Although the nematode has many advantages as a model system, it is not the optimum model for evaluating vertebrate cell death activity or for use in screening compounds for potential therapeutic activity in vertebrates.
There are currently two approaches for detecting cell death activity in vertebrate hosts. The first approach uses standard cell culture techniques and typically relies on standard microplate readers to detect the death of cells cultured from an organism. A major drawback of the cell culture assay format is that it does not permit analysis of the effects of a compound on cell types that have not been cultured (i.e., other cell types). It also does not allow evaluation of the effects of a compounds on specific tissues or organs or in an intact whole host in vivo. Furthermore, such an assay format does not permit the monitoring of cell death activities in multiple tissues, organs, or systems of a live host simultaneously or the continued monitoring of such activities over time. In addition, the cell culture assay approach does not allow for rapid or automated high-throughput screening of many compounds.
A second approach to detecting cell death activity utilizes a histochemical staining technique, designated terminal deoxyuridine nucleotide end labeling (TUNEL), to detect dead or dying cells in sectioned tissues of vertebrate embryos. Unfortunately, with this approach, only a single time point in the life cycle of the host can be examined; the death of cells in various tissues or organs of the subject over a period of time cannot be monitored. Because many degenerative diseases occur in stages, the ability to examine changes in the pattern of cell death activity caused by a compound and the duration of direct and side effects of the compound on multiple tissues and organs would represent a significant improvement over such methods. Moreover, because the TUNEL approach requires that cells be fixed for visualization, effects in a live animal cannot be monitored.
The identification of target-specific therapeutic and prophylactic compounds that exhibit low toxicity and/or side effects has also been focal point of drug discovery and development. Evaluation of the potential impact of different compounds on humans and animal health is a major component of risk assessment. There is increasing concern that current toxicity test procedures are inadequate. A number of cell-based in vitro toxicity screens have been developed. Significantly, however, these screens do not permit evaluation of the toxic effects of a compound in vivo on an intact animal. Notably, these cell-based assays are designed at the molecular and cellular levels; as a result, determining the impact of a compound of interest on higher levels of cellular organization, such as the circulatory system and neurodevelopment, still requires subsequent whole animal testing. In addition, current screens do not permit the assessment of drug effects on all potential target cells, tissues, or organs of an animal. Nor can the effects of a compound on multiple target tissues and organs be studied simultaneously or over time using current assays. Underscoring the need for the development of more predictive and comprehensive toxicity screening methods, many compounds that pass preliminary cell-based testing fail final large animal toxicity testing, a prerequisite for eventual FDA approval. Furthermore, some potential therapeutic compounds that do not produce immediate lethality induce toxic effects in specific organs and tissues. There is a need for a cost-effective, comprehensive methods for screening compounds for toxic activity in whole animals and in one or more designated target tissues and organs in vivo and in cells in vitro and over time.